For most investors running a taxable equity portfolio, the Alternative Minimum Tax is a background concern that rarely changes how harvested losses work. Capital gains and losses use the same preferential rates under both the regular tax and AMT computations, so harvested losses can offset long-term gains the same way under either system.
The edge cases are narrower but genuinely important. Incentive stock options create a basis divergence that can make a "loss" under regular tax look like a "gain" under AMT — and vice versa. State-level AMT adds another layer in California and a handful of other states. And AMT timing may change the marginal value of a harvest when short-term gains are involved. This article maps each case.
What is the AMT and who still faces it under current law?
What is the Alternative Minimum Tax, and which taxpayers are most likely to owe it after the Tax Cuts and Jobs Act of 2017?
The AMT is a parallel tax computation — taxpayers owe whichever amount is higher, regular tax or AMT — and since TCJA substantially raised exemption thresholds to approximately $85,700 for single filers and approximately $133,300 for married couples filing jointly (2024 figures, indexed annually for inflation), it most commonly applies today to taxpayers with incentive stock option spreads, depreciation differences, or other preference items that are added back under AMT rules.
Under the AMT computation, taxpayers start from adjusted gross income and add back certain "preference items" — income or deductions the regular code favors but AMT does not — to arrive at Alternative Minimum Taxable Income (AMTI). After subtracting the applicable exemption, two rates may apply: approximately 26% on the first roughly $220,700 of net AMTI above the exemption (2024, indexed annually), and approximately 28% on any AMTI above that threshold. The result is compared to the regular tax; if AMT is higher, the difference is owed on top of the regular bill.
Before the Tax Cuts and Jobs Act of 2017, the AMT affected a much wider population because exemption amounts were not indexed and had grown stale over decades. Today the group most commonly affected includes employees who exercise large incentive stock option grants in a single year, taxpayers with large depreciation differences between regular and AMT accounting, and — in certain states — higher-income residents where state-level AMT persists even when the federal AMT no longer applies.
Do harvested capital losses work the same way under AMT?
Do harvested capital losses offset gains identically under the AMT computation as under the regular income tax?
For most stock-and-fund portfolios, yes — long-term capital gains use the same preferential rates (historically 0%, 15%, or 20% depending on income level) under both regular tax and AMT, and harvested losses can reduce those gains identically under both systems; the main exceptions involve short-term gains, which may be taxed at different effective rates in an AMT year, and ISO positions where AMT and regular-tax basis may diverge.
A common misconception is that the AMT overrides the preferential capital gains rates and taxes all gains at a higher flat rate. It does not. Under IRC §55(b), the capital gains preference was repealed and long-term capital gains are explicitly taxed at the same preferential rates under both the regular and AMT computations.
Where the systems may diverge for regular equity investors: short-term capital gains are included in AMTI as ordinary income. In a year when AMT applies, those gains may be taxed at the AMT rate (up to approximately 28%) rather than the top regular marginal rate (up to approximately 37%). Harvesting losses to offset short-term gains in an AMT year may therefore produce somewhat different tax savings than a regular-rate analysis suggests — potentially less, if the regular marginal rate would have been higher.
For a foundation on how harvested losses interact with the broader tax calculation, see TLH 101 and tax alpha explained.
The ISO option edge case: when AMT and regular-tax basis diverge
Why do incentive stock options create a unique AMT basis problem when a position later shows a loss?
When an ISO is exercised, the spread between the exercise price and fair market value at exercise is an AMT preference item added to AMTI in that year — giving the shares a higher AMT basis (approximately the FMV at exercise) than their regular-tax basis (the exercise price) — so if the stock later falls below the exercise price, the position may show a loss under regular tax while showing no loss, or even a gain, under AMT.
An example makes the mechanics concrete. Suppose an employee exercises ISOs with an exercise price of approximately $20 per share when the stock trades at around $80. Under regular tax, the cost basis is approximately $20 per share. Under AMT, the roughly $60 spread may be added to AMTI in the exercise year, establishing an AMT basis of approximately $80 per share. If the stock later falls to around $30:
- Under regular tax: a sale at approximately $30 may produce a gain of roughly $10 (approximately $30 sale price less the approximately $20 regular-tax basis).
- Under AMT: the same sale may produce a loss of approximately $50 (approximately $30 sale price less the approximately $80 AMT basis).
In the opposite direction — if the stock falls to around $15 — a regular-tax sale may produce a loss of approximately $5, while under AMT the same sale may produce a loss of roughly $65. The two loss amounts may be usable in different ways, and the tax outcome depends partly on which year the position is sold and whether the taxpayer is in an AMT year at that time.
When AMT paid in the exercise year exceeds regular tax, the excess carries forward as an AMT credit (Form 8801) and may reduce regular tax in future non-AMT years. How and when that credit is used affects the net economics of the entire ISO-exercise-and-harvest sequence.
For investors holding ISO positions, maintaining separate records for regular-tax basis and AMT basis per lot is the foundation of any AMT-aware harvest analysis. See the IRS code cheat sheet for context on how basis rules interact across the broader tax code.
How AMT timing affects the value of a harvest
How does being in an AMT year change the economic value of tax-loss harvesting decisions?
In years when AMT applies, harvested losses offsetting long-term gains may produce the same tax benefit as in any other year (since long-term capital gains rates are identical under both systems), but losses offsetting short-term gains may produce different savings because AMT rates on ordinary income (approximately 26–28%) may differ from the investor's regular marginal rate — and harvests that reduce AMTI in an AMT year may also affect the size of AMT credits that could otherwise carry forward to future years.
The AMT credit dynamic is worth understanding. In a year when AMT exceeds regular tax, the taxpayer pays AMT and the excess creates a credit under Form 8801. That credit carries forward indefinitely and may reduce regular tax in future years when AMT no longer applies.
A harvest in an AMT year may reduce AMTI and therefore reduce the current-year AMT bill — which is generally valuable — but it can also reduce the size of the AMT credit being generated. Whether reducing the current-year AMT or preserving AMT credits for future use produces a better outcome depends on the expected future tax path. Many investors with recurring ISO exercises find it useful to model multiple tax years before making large harvest decisions, rather than optimizing each year in isolation.
For strategies that coordinate multiple elements of the tax picture in a single year — bracket management, ordinary-income timing, and capital gains — see Roth conversions and tax-loss harvesting, which covers how these tools can interact within a single tax year.
State-level AMT: California and other states
Does California have its own Alternative Minimum Tax, and how does it interact with the federal AMT for harvest decisions?
California maintains its own AMT at an approximately 7% rate with a different exemption structure from the federal computation, and unlike the federal AMT, California's system was not substantially restructured by TCJA — meaning California residents may still face state AMT even in years when they no longer owe the federal AMT.
California's AMT starts from California taxable income and adds back many of the same preference items as the federal computation (ISO spreads, depreciation differences) but uses its own exemption and phase-out thresholds. For California residents, the ISO basis divergence issue described above may apply at both the federal and state levels, with California using its own AMT basis adjustment rules.
Investors with significant ISO exercises in a single year may potentially face both federal and California AMT simultaneously — generating credits usable against future California regular tax as well as federal credits. The credits operate separately under the two systems and carry forward independently.
New Jersey uses a modified alternative minimum assessment approach; several other states maintain some version of an AMT with varying rates and preference items. The specifics vary significantly, and state-level AMT calculations are generally not part of standard direct-indexing software. Working with a tax professional who practices in the relevant state is the appropriate approach for state AMT analysis.
What this means for tax-aware portfolio decisions with AMT exposure
How can investors with AMT exposure factor it into their harvest planning?
Investors with ISO stock options or other substantial AMT preference items may benefit from maintaining separate AMT and regular-tax basis records per lot, mapping expected AMT exposure across multiple years at the start of harvest season, and evaluating whether a proposed harvest produces consistent results under both computations — particularly for positions where the two basis figures diverge materially.
For the majority of investors in a standard direct-index or stock-plus-ETF portfolio without ISO exposure, AMT does not materially change the harvest decision. Long-term gains are taxed at the same rates under both systems; the standard harvest framework applies unchanged.
For investors with ISO exposure, the practical steps many commonly take include: tracking AMT and regular-tax basis separately at the lot level; modeling AMT liability before the harvest window closes; and identifying positions where basis divergence is large enough to produce meaningfully different results under the two systems. Some investors spread recognition events across AMT and non-AMT years where the calendar allows.
HarvestEngine's lot selection logic tracks per-lot basis and supports custom basis entries that allow AMT-aware clients to reflect AMT basis where it differs from regular-tax cost basis. The broader harvesting strategy discussion continues in the zero-tax exit strategy article and the wash-sale rule explainer.